The present invention relates to a liquid crystal cell array which can be applied to a liquid crystal light shutter capable of modulating the light-transmitting (or bright) area by the operation of individual liquid crystal cells. The invention also pertains to a method for driving such a liquid crystal cell array.
Heretofore, liquid crystal has been applied not only to character and image displays but also to a light shutter which controls the transmission therethrough of light. In the light shutter the transmission of light through the liquid crystal is usually subjected to ON-OFF control on a binary basis. The present inventors produced a nematic liquid crystal cell with a metal electrode deposited on either side of a transparent electrode, and proposed a driving method therefor according to which a potential gradient is produced in the transparent electrode by applying different voltages to the metal electrodes to form a light-transmitting and a light-intercepting areas (this operation will hereinafter be referred to as the diaphragming operation) and the ratio between these areas can freely be varied by controlling the potential gradient (Japanese Patent Application No. 181,912/86). The principle of the driving method will be described with reference to FIG. 1.
FIG. 1 is an exploded perspective view, partly cut away, of a liquid crystal cell 11 which is used for forming the above-mentioned liquid crystal light shutter. The liquid crystal cell 11 is made up of a spacer 4 and a pair of opposed transparent substrates 2 and 3 with the spacer 4 interposed therebetween and contains liquid crystal in the space defined by the two transparent substrates 2 and 3 and the spacer 4. The one transparent substrate 2 (hereinafter referred to also as the first substrate) has a transparent electrode 9 deposited on the inner surface thereof and a metal electrode 7 deposited on one marginal portion of the transparent electrode 9. The other transparent substrate 3 (hereinafter referred to also as the second substrate) also has a transparent electrode 8 deposited on the inner surface thereof and metal electrodes 5 and 6 deposited in parallel on both marginal portions of the transparent electrode 8. A pair of opposed polarizing plates (not shown) are disposed with such a liquid crystal cell sandwiched therebetween, forming a light shutter. The area over which light is transmitted through the transparent electrodes 2 and 3 of the liquid crystal cell 11 is called a cell window. In general, the transparent electrodes 8 and 9 are formed by transparent conductive films of the indium oxide or tin oxide series and the metal electrodes 5 to 7 are formed by vapor deposition of aluminum, nickel or chromium, but these electrodes are not limited specifically to them and may be formed of any materials so long as they are lower in resistance than the transparent electrodes 8 and 9 and capable of intercepting light. The following description of the prior art will be given of the case where the liquid crystal sealed in the cell 11 is ferroelectric liquid crystal. Since the ferroelectric liquid crystal changes its orientation from one to the other upon inversion of the polarity of the applied voltage, it is possible to determine the direction of polarization of the polarizing plates relative to the liquid crystal cell 11 so that it permits or inhibits the passage therethrough of light in response to a desired one of the polarities of the applied voltage.
Next, the basic operation of the liquid crystal cell 11 of the above arrangement will be described with regard to the drawings. The inner marginal edges (hereinafter referred to as electrode edges) of the metal electrodes 5 and 6 on the second substrate 3 in FIG. 1 are indicated by a and b, respectively. When voltages V.sub.1 and V.sub.2 are applied to the metal electrodes 5 and 6, respectively, if the voltages V.sub.1 and V.sub.2 are not equal, then current will flow through the transparent electrode 8 across the metal electrodes 5 and 6. At this time, the transparent electrode 8 serves as a resistor, and accordingly, a substantially linear potential gradient develops in the transparent electrode 8 between the electrode edges a and b. This is shown in FIG. 2A, in which the solid line (i) indicates the potential gradient developed between the electrode edges a and b. On the other hand, when a voltage V.sub.3 is fed to the metal electrode 7 of the first substrate 2, the potential on the transparent electrode 9 will become constant over the area of its surface as indicated by the solid line (ii) in FIG. 2A. As a result of this, a voltage corresponding to the difference between the solid lines (i) and (ii), that is, a voltage corresponding to the potential difference is provided between the two transparent electrodes 8 and 9 and this voltage is applied to the liquid crystal sandwiched between them.
Now, the direction of the polarizing plates is predetermined so that light is intercepted when the potential of the transparent electrode 8 is higher than the potential of the transparent electrode 9. When the voltages V.sub.1, V.sub.2 and V.sub.3 are applied to the metal electrodes 5, 6 and 7 with the relationships V.sub.1 &gt;V.sub.3 &gt;V.sub.2, there are formed, on one side of the intersecting point K of the solid lines (i) and (ii), an area Ka where the potential of the second substrate 3 is higher than the potential of the first substrate 2 and, on the other side, an area Kb where the potential of the second substrate 3 is lower than the potential of the first substrate 2. In this instance, only the area Ka transmits therethrough light but the area Kb intercepts it. FIG. 2B shows the front elevation of the cell window in this state. In FIG. 2B reference numeral 41 indicates the light-transmitting area of the cell window and 42 the light-intercepting area. The operation (or the state) of the liquid crystal by (or in) which light is permitted to pass through a portion of the cell window as described above will hereinafter be referred to as the "diaphragming operation (or state)" and the ratio of the light-transmitting area to the total area of the cell window as the "aperture ratio". Further, the liquid crystal cell which is capable of controlling the aperture ratio of the cell window by the voltage gradient as mentioned above will hereinafter be called the gradient voltage drive liquid crystal cell.
The aperture ratio of the cell window can be changed by suitably selecting the voltages V.sub.1, V.sub.2 and V.sub.3 in FIG. 2A, for example. When the voltage V.sub.3 on the side of the first substrate 2, that is, the voltage which is applied to the transparent electrode 9, is decreased as indicated by the solid line (ii) in FIG. 2C, the intersecting point K of the solid lines (i) and (ii) shifts toward the electrode edge b, increasing the light-transmitting area 41 and decreasing the light-intercepting area 42. Conversely, when the voltage V.sub.3 is increased in FIG. 2A, the light-transmitting area 41 decreases and the light-intercepting area 42 increases. It is apparent that the aperture ratio can also be varied by changing the values of the voltages V.sub.1 and/or V.sub.2 while keeping the voltage V.sub.3 at a fixed value. Such a diaphragming operation of the cell window is not limited according to the type of the cell used but can be achieved for a birefringence controlling type liquid crystal cell, a TN type liquid crystal cell, a guest-host type liquid crystal cell, a two-frequency driving type liquid crystal cell, etc. as well as the ferroelectric liquid crystal cell.
In a light shutter in which a number of such gradient voltage drive liquid crystal cells 11 as shown in FIG. 1 are simply arrayed, it is necessary to provide wiring on the second substrate 3 for sending at least two kinds of drive voltage signals to each liquid crystal cell. This constitutes an obstacle to high density packaging of cell arrays. Moreover, at least two drivers must be provided for each gradient voltage drive liquid crystal cell; this also enlarges the scale of the light shutter and raises its cost accordingly.